Tunable sound transmission shapes up

We often make use of the phenomena that sound travels well through solids. The new crystal structures could provide unprecedented control over the types of sound waves that can propagate along their surfaces.
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The ability to control fine-scale acoustic waves known as phonons could lead to new sensing and surgery technologies, or even materials that are invisible to sonar. This pursuit led KAUST researchers to describe new phononic crystals whose properties can be tuned to control the propagation of different frequencies of phonons.

Ying Wu from the University’s Computer, Electrical and Mathematical Science and Engineering Division explains that the researchers' work was inspired by a phenomenon called topological insulation, which was first observed in electronic systems.

Conventionally, if a material is an insulator, then no part of it will conduct electricity. This logic was turned on its head by the discovery of topological insulators, which have insulating interiors but conducting states on their surfaces that can be manipulated for applications such as quantum computing.

Researchers have extended the concept of topological insulation to materials that only allow certain light or sound waves to propagate on their surfaces. Specifically, these materials may break a physical property called time-reversal symmetry (TRS) to produce a bandgap, a range of frequencies at which no phonons can be transmitted.

“Compared to quantum and electromagnetic systems, acoustic systems did not draw much attention because TRS in acoustic systems is intrinsically conserved,” said Wu. “This situation changed in 2015, when several methods were proposed to break TRS in acoustic systems, such as introducing rotating fluids.”

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